Activated carbon, method for production thereof and use thereof

ABSTRACT

A method for producing an activated carbon material, wherein the method comprises a step of thermally treating coal-based pitch at two temperature ranges of 400° C. to 600° C. and 600° C. to 900° C.; and a step of mixing the thus obtained carbonaceous material with an alkali metal compound and effecting activation thereof at 600° C. to 900° C., and an activated carbon material obtained by the method. When the activated carbon material of the present invention is used a polarizable electrode material of an electric double layer capacitor, high capacitance per electrode is attained without application of excessive voltage. By adding fibrous material to a coal-based pitch during activation expansion of an alkali molten liquid can be suppressed and productivity can be drastically improved. Furthermore, employment of an fibrous carbon material which is excellent in conductivity as a fibrous material, carbon fiber is melt-bonded on the surface of the activated carbon material, which enables production of a polarizable electrode exhibiting excellent charge/discharge characteristics at high current density.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based on the provisions of 35 U.S.C. Article 111(a)with claiming the benefit of filing dates of U.S. provisionalapplication Ser. No. 60/318,623 filed on Sep. 13, 2001 and No.60/380,858 filed on May 17, 2002 under the provisions of 35 U.S.C.111(b), pursuant to 35 U.S.C. Article 119(e)(1).

TECHNICAL FIELD

The present invention relates to an activated carbon material which canbe employed in a variety of uses including treatment of tap water orwastewater, and as catalyst carrier, gas occlusion material, electrodematerial for an electric double layer capacitor (also called “electricdouble layer condenser”) and the like as well as to a method forproducing the activated carbon material. The present invention alsorelates to a polarizable electrode containing the activated carbonmaterial and to an electric double layer capacitor containing theelectrode and exhibiting high capacitance and high durability.

BACKGROUND ART

Carbon material, particularly activated carbon material, is employed ina variety of fields; for example, it finds utility in treatment ofwater, catalyst carrier, gas occlusion and electric double layercapacitor electrodes. Among these, an electric double layer capacitorexhibits, for example, the following characteristics: rapid charging anddischarging; resistance to excessive charging and discharging; longservice life (since it does not undergo chemical reaction); a widetemperature range in which the capacitor can be used; andenvironmentally friendly nature (since it contains no heavy metal).Therefore, conventionally, an electric double layer capacitor has beenemployed in, for example, a memory backup power supply. In recent years,electric double layer capacitors of high capacitance have been developedrapidly, and such electric double layer capacitors have been employed inhigh-performance energy devices. Furthermore, an electric double layercapacitor has been envisaged to be employed in a power storage system incombination with a solar battery or a fuel cell, or to be employed forassisting a gasoline engine of a hybrid car.

An electric double layer capacitor includes a pair of positive andnegative polarizable electrodes formed of, for example, activatedcarbon, the electrodes facing each other with the intervention of aseparator in a solution containing electrolyte ions. When DC voltage isapplied to the electrodes, anions contained in the solution migrate tothe positively polarized electrode, and cations contained in thesolution migrate to the negatively polarized electrode. Electric energyis obtained from an electric double layer formed at the interfacebetween the solution and each of the electrodes.

Conventional electric double layer capacitors are excellent in powerdensity but poor in energy density. Therefore, in order to realizeemployment of such an electric double layer capacitor in energy devices,capacitance of the capacitor must be increased further. In order toincrease capacitance of an electric double layer capacitor, an electrodematerial which enables formation of a large number of electric doublelayers in an electrolytic solution must be developed.

An electrode predominantly containing activated carbon material isemployed as a component constituting an electric double layer capacitor.Such activated carbon material is required to exhibit, as a keyfunction, high capacitance per mass or per volume.

In view of the foregoing, use of an activated carbon material having alarge specific surface area has been contemplated as an electrodematerial which enables formation of a large number of electric doublelayers. When such an activated carbon material is employed, capacitanceper mass (F/g) increase, but capacitance per volume (F/ml) fails toincrease to an intended level, because of lowering of electrode density.

In recent years, there has been proposed an approach to production of anactivated carbon material containing microcrystals similar to those ofgraphite, along with employment of the thus-produced activated carbonmaterial as a raw material for forming a polarizable electrode (JapanesePatent Application Laid-Open (kokai) No. 11-317333). In view that anelectric double layer capacitor including a polarizable electrode formedfrom the activated carbon material exhibits high capacitance, theactivated carbon material is considered an excellent electrode material.

However, the aforementioned activated carbon material is not necessarilysatisfactory, in that it involves some problems. That is, sinceexpansion of the activated carbon material occurs during application ofvoltage, as described in the above publication, a size-limitingstructure must be provided for suppressing expansion of the activatedcarbon material, and thus difficulty is encountered in assembling acapacitor. In addition, application of a voltage of as high as about 4 Vis required in advance in order to obtain sufficient capacitance of thecapacitor. As a result, decomposition of an electrolytic solution mayoccur.

In a typical method for producing activated carbon material, an organicsubstance such as coconut shell, pitch, or phenol resin is thermallydecomposed to thereby yield a carbonized material, and the carbide isactivated.

Examples of activation methods include gas activation employing steam orcarbon dioxide gas, and chemical activation employing, for example,potassium sulfide, zinc chloride, or an alkali hydroxide. Particularly,activation employing an alkali hydroxide such as potassium hydroxide orsodium hydroxide is effective for producing an activated carbon materialhaving a large specific surface area, and an activated carbon materialproduced through this activation method exhibits high capacitance permass or per volume.

When alkali activation, for example, activation employing an alkalihydroxide, is employed, an alkali metal compound is melted throughheating, and a carbon material is impregnated and reacted with themolten alkali metal compound, to thereby form a porous structure andactivate the carbon material. When alkali activation of powdery orgranular carbon raw material is employed in a container such as acrucible, effervescence of a molten liquid occurs during activation, dueto generation of, for example, moisture or hydrogen gas, and the moltenliquid may overflow the container. Particularly when alkali activationis carried out at high temperature increase rate, the amount of gasgenerated in a unit time increases, and overflow of the molten liquidtends to occur. In order to avoid such overflow, the amounts of analkali metal compound and a carbon raw material which are placed in acontainer must be limited, and therefore productivity of analkali-activated carbon material is considerably lowered, resulting inhigh production cost.

Accordingly, an object of the present invention is to provide anactivated carbon material which enables capacitance per electrode to beincreased without application of a high voltage.

Another object of the present invention is to drastically improve theproductivity in the activation of an activated carbon material and toproduce an activated carbon material which is excellent in capacitanceat high current density.

SUMMARY OF THE INVENTION

The present invention has been accomplished as a result of extensiveinvestigations for solving the aforementioned problems and provides anactivated carbon material, the production method and the use thereofshown below.

-   1. A method for producing an activated carbon material, wherein the    method comprises a step of thermally treating coal-based pitch at    two temperature ranges of 400° C. to 600° C. and 600° C. to 900° C.;    and a step of mixing and heating the thus-treated coal-based pitch    with an alkali metal compound for the activation thereof.-   2. The method for producing an activated carbon material according    to 1 above, wherein the alkali metal compound is at least one alkali    hydroxide selected from the group consisting of sodium hydroxide,    potassium hydroxide, and cesium hydroxide.-   3. The method for producing an activated carbon material according    to 1 above, wherein the step of thermally treating coal-based pitch    at two temperature ranges is carried out in a vapor of an alkali    metal.-   4. The method for producing an activated carbon material according    to 3 above, wherein the alkali metal compound is at least one    species selected from the group consisting of potassium, sodium, and    cesium compounds.-   5. The method for producing an activated carbon material according    to 1 above, wherein the step for the activation comprises adding a    fibrous material to the coal-based pitch.-   6. The method for producing an activated carbon material according    to 5 above, wherein the amount of the fibrous material is not less    than 0.05 mass % as a corresponding mass of the fibrous material    heated at 800° C. in an inert atmosphere on the basis of the    coal-based pitch.-   7. The method for producing an activated carbon material according    to 5 or 6 above, wherein the outer diameter of each fiber filament    of the fibrous material is 1000 nm or less.-   8. The method for producing an activated carbon material according    to any one of 5 to 7 above, wherein the fibrous material is a    material capable of maintaining its shape up to at least 300° C.-   9. The method for producing an activated carbon material according    to any one of 5 to 8 above, wherein the fibrous material is at least    one species selected from the group consisting of a fibrous carbon,    carbonized material of organic fiber, unmeltable fiber, beaten pulp    and cellulose fiber.-   10. The method for producing an activated carbon material according    to 9 above, wherein the fibrous carbon is at least one species    selected from the group consisting of a carbon nano tube, whiskers,    vapor grown carbon fiber, carbon fiber ribbon and coiled carbon    fiber.-   11. The method for producing an activated carbon material according    to 10 above, wherein each fiber filament of the vapor grown carbon    fiber contains a hollow space extending along its center axis, and    has an outer diameter of 2 to 500 nm and an aspect ratio of 10 to    15,000.-   12. The method for producing an activated carbon material according    to 11 above, wherein the vapor grown carbon fiber is branched carbon    fiber.-   13. An activated carbon material produced through a production    method as recited in any one of 1 to 12 above.-   14. The activated carbon material according to 13 above, which has a    BET specific surface area of 10 to 1,000 m²/g as measured by means    of a nitrogen adsorption method, and contains no graphite    microcrystals.-   15. The activated carbon material according to 13 or 14 above,    wherein the ratio of the height of the D peak (1,360 cm⁻¹) of a    Raman spectrum of the activated carbon material to that of the G    peak (1,580 cm⁻¹) of the Raman spectrum is 0.8 to 1.2.-   16. The activated carbon material according to any one of 13 to 15    above, wherein pores of the activated carbon material having a size    of 20 to 50 Å as measured by means of a BJH method employing    nitrogen adsorption have a pore volume of at least 0.02 ml/g.-   17. A polarizable electrode material comprising a activated carbon    material as recited in any one of 13 to 16 above and optionally    vapor grown carbon fiber.-   18. The polarizable electrode material according to 17 above,    wherein the amount of the vapor grown carbon fiber is 0.05 to 50    mass %.-   19. The polarizable electrode material according to 17 or 18 above,    wherein each fiber filament of the vapor grown carbon fiber contains    a hollow space extending along its center axis, and has an outer    diameter of 2 to 500 nm and an aspect ratio of 10 to 15,000.-   20. The polarizable electrode material according to any one of 17 to    19 above, wherein the vapor grown carbon fiber contains micropores    having a pore volume of 0.01 to 0.4 ml/g, and has a BET specific    surface area of 30 to 1,000 m²/g as measured by means of a nitrogen    adsorption method.-   21. An electric double layer capacitor comprising a polarizable    electrode prepared from a polarizable electrode material as recited    in any one of 17 to 20 above.-   22. A method for producing an activated carbon material, wherein the    method comprises adding an alkali metal compound as an activating    agent and a fibrous material to a carbonaceous raw material and    heating the mixture.-   23. The method for producing an activated carbon material according    to 22 above, wherein the amount of the fibrous material is not less    than 0.05 mass % as a corresponding mass of the fibrous material    heated at 800° C. in an inert atmosphere on the basis of the    carbonaceous raw material.-   24. The method for producing an activated carbon material according    to 22 or 23 above, wherein the outer diameter of each fiber filament    of the fibrous material is 1000 nm or less.-   25. The method for producing an activated carbon material according    to any one of 22 to 24 above, wherein the fibrous material is a    material capable of maintaining its shape up to at least 300° C.-   26. The method for producing an activated carbon material according    to any one of 22 to 25 above, wherein the fibrous material is at    least one species selected from the group consisting of a fibrous    carbon, carbonized material of organic fiber, unmeltable fiber,    beaten pulp and cellulose fiber.-   27. The method for producing an activated carbon material according    to 26 above, wherein the fibrous carbon is at least one species    selected from the group consisting of a carbon nano tube, whiskers,    vapor grown carbon fiber, carbon fiber ribbon and coiled carbon    fiber.-   28. The method for producing an activated carbon material according    to 27 above, wherein each fiber filament of the vapor grown carbon    fiber contains a hollow space extending along its center axis, and    has an outer diameter of 2 to 500 nm and an aspect ratio of 10 to    15,000.-   29. The method for producing an activated carbon material according    to 28 above, wherein the vapor grown carbon fiber is branched carbon    fiber.-   30. An activated carbon material having a fibrous material fused    onto at least a portion of the surface of the activated carbon    material particle.-   31. The activated carbon material according to 30 above, which    assumes a spherical shape.-   32. An activated carbon material produced through a method for    producing an activated carbon material as recited in any one of 22    to 29 above.-   33. A polarizable electrode comprising, as an electrode material, an    activated carbon material as recited in any one of 30 to 32 above.-   34. An electric double layer capacitor comprising a polarizable    electrode as recited in 33 above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing a cell employed for evaluatingan electric double layer capacitor.

FIG. 2 shows a Raman spectrum of the activated carbon material producedin Example 1. The vertical axis corresponds to spectrum intensity(Int.), and the horizontal axis corresponds to Raman shift (measurementwavelength: cm⁻¹).

FIG. 3 shows a transmission electron microscope (TEM) photograph of theactivated carbon material produced in Example 3 (magnification:×2,000,000).

FIG. 4 shows an electron micrograph of the activated carbon materialproduced in Example 6 (magnification: ×5,000).

DESCRIPTION OF THE INVENTION

Electric characteristics of an activated carbon material greatly varywith structural characteristics, including specific surface area, poredistribution, and crystal structure of the activated carbon material.Such structural characteristics of the activated carbon material aredetermined on the basis of the structure of a raw material,carbonization conditions, and activation conditions.

Therefore, in order to produce an activated carbon material useful as anelectrode material, the structure of a raw material, carbonizationconditions, and activation conditions must be optimized. The presentinventors have considered that coal-based pitch is suitably employed asa raw material for producing an activated carbon material. As comparedwith a petroleum-based carbon raw material, coal-based pitch has a smallnumber of side chains, contains aromatic compounds at high proportions,and contains polycyclic aromatic compounds of different molecularstructures. Therefore, when an activated carbon material is producedfrom coal-based pitch, conceivably, a variety of complicatedmicrocrystalline structures derived from such compounds are formed inthe activated carbon material, and thus the activated carbon materialexhibits excellent electric characteristics.

No particular limitation is imposed on the coal-based pitch which may beemployed. However, coal-based pitch having a softening point of 100° C.or lower is preferred, and coal-based pitch having a softening point of60° C. to 90° C. is more preferred.

Such coal-based pitch is subjected to two-stage heat treatment includingfiring and carbonization at temperature ranges of 400 to 600° C. and 600to 900° C., preferably 450 to 550° C. and 650 to 850° C.

When coal-based pitch is heated at 400 to 600° C., thermal decompositionreaction proceeds, gas and light components are removed from the pitch,polycondensation of the residue occurs, and finally the pitch issolidified. In this first-stage carbonization step, the state ofmicroscopic bonding between carbon atoms is substantially determined,and the crystalline structure determined in this step determines thefundamental structure of an activated carbon material (i.e., a finalproduct).

In the first-stage carbonization step, the temperature increase rate ispreferably 3 to 10° C./hour, more preferably 4 to 6° C./hour; and themaintenance time at the maximum temperature is preferably 5 to 20 hours,more preferably 8 to 12 hours.

Subsequently, the second-stage heat treatment is carried out at atemperature range of 600 to 900° C. In this second-stage carbonizationstep, the temperature increase rate is preferably 3 to 10° C./hour, morepreferably 4 to 6° C./hour; and the maintenance time at the maximumtemperature is preferably 5 to 20 hours, more preferably 8 to 12 hours.

The above heat treatment (carbonization) steps are effectively carriedout in a vapor of an alkali metal. An alkali metal serves as a catalystin the carbonization step. That is, an alkali metal promotescross-linking between aromatic compounds contained in the pitch, tothereby allow carbonization to proceed. Examples of the alkali metalsinclude compounds of sodium, potassium and cesium.

The heat treatment method in a vapor of an alkali metal can beconducted, for example, by heating the carbonization step system whileintroducing to the system a vapor of an alkali metal vaporized from thealkali activation reaction system described below. Alternatively, theheat treatment step can be conducted by placing pitch material aroundthe vessel for the reaction alkali activation reaction to expose thepitch material to an alkali metal vapor vaporized from the alkaliactivation reaction system while heating the pitch material, therebyconcurrently effecting the heat treatment (carbonization) and alkaliactivation reaction steps. This method shortens the total treatment timeand also reduces the cost for heating.

Subsequently, the carbonized material (thermally treated carbonaceousmaterial) is subjected to pulverization so as to attain a particle sizeof about 1 to about 100 μm, and the thus-pulverized product is mixedwith an alkali metal compound and then heated so as to form pores in thecarbonized product, thereby producing an activated carbon material.

In an activation method employing an alkali metal compound (an alkaliactivation method), a carbonaceous raw material (e.g., a carbonizedmaterial) is uniformly impregnated with an alkali metal compound, andthe carbonaceous raw material is heavily corroded by the alkali metalcompound under heating (firing), to thereby produce an activated carbonmaterial having an intricately developed porous structure.

The activation step is preferably conducted with a fibrous materialmixed with the raw material. Expansion of an alkali molten liquid can besuppressed by mixing a fibrous material, which enables the productivityto be improved. Furthermore, the fibrous carbon exhibiting excellentelectrical conductivity (e.g., vapor grown carbon fiber) is melt-bondedonto the surfaces of the resultant activated carbon particles, andcontact resistance between the activated carbon particles can bereduced. As a result, when the activated carbon is used as a polarizableelectrode for an electric double layer capacitor, properties such ascapacitance holding properties (cycle properties) thereof are improved.

Similar effects can be obtained by using other carbonaceous materials inplace of a coal-based pitch as a carbonaceous raw material in the alkaliactivation method comprising mixing of a fibrous material.

No particular limitation is imposed on the other carbonaceous materialemployed in the production method of the present invention. Examples ofthe carbonaceous raw material which may be employed include productsobtained through carbonization of coconut shell, coffee bean, lignin,sawdust, polyvinylidene chloride, phenol resin, coal, coal tar, coalcoke, and petroleum coke; PAN-based carbon fiber; and pitch-based carbonfiber. Carbonization of a carbonaceous raw material can be carried outin a method other than two-stage heating. A carbonaceous raw material iscarbonized typically at 400 to 1,000° C. but a material which has notundergone carbonization may be mixed with an alkali metal compound. Whena carbonaceous raw material is carbonized at a temperature higher than1,000° C., the activation rate of the resultant material is lowered, andactivation of the material requires a long period of time. The particlesize of a carbonaceous raw material may be that of a 10-mesh sieve (ASTMstandards, mesh size: 2.0 mm) or rougher one. However, the particle sizeof a carbonaceous raw material is preferably that of a residue obtainedthrough sieving by use of a sieve of 10 mesh or less, more preferably,50 mesh (mesh size: 0.297 mm) or less, much more preferably 100 mesh(mesh size: 0.149 mm) or less.

No particular limitation is imposed on the fibrous carbon usable in thepresent invention, so long as the fibrous carbon can maintain its shapeup to 300° C. and hold an alkali molten liquid. Examples of the fibrouscarbon which may be employed include a fibrous carbon (e.g. carbon nanotube, whiskers, vapor grown carbon fiber, carbon fiber ribbon and coiledcarbon fiber), beaten pulp products, cellulose fibers (e.g. naturalfiber, regenerated cellulose), carbonized material of organic fiber(e.g., PAN) and unmeltable fiber. These fibers may be employed incombination of two or more species. The term “unmeltable fiber” as usedherein refers to a fiber made from a spun fiber such as melt spinningfiber, centrifugal spinning fiber and the like which has been subjectedto a heat treatment in an oxidizing atmosphere such as air and oxygenthereby provided with bridges between fiber constituting molecules andthus prevented from getting out of its shape during the subsequent heattreatment.

No particular limitation is imposed on the shape of the fibrous carbon,so long as the fibrous carbon can suppress expansion of an alkali moltenliquid during activation. Examples of the shape of the fibrous carboninclude a ribbon-like shape which has a flat cross section, a coiledshape (e.g. coil, spiral, helix and spring), carbon spring, carbonmicrocoil, helical polyacetylene and the like.

The reason why expansion of an alkali molten liquid can be suppressed byaddition of fibrous carbon has not yet been fully elucidated.Conceivably, an alkali molten liquid is held in fibrous carbon. Branchedfibrous carbon of small diameter exerts the effect of suppressingexpansion of an alkali molten liquid. The outer diameter of fibrouscarbon to be employed is typically 1000 nm or less, preferably 500 nm orless, more preferably 10 nm or more and 400 nm or less. Those which havebeen fibrilized (i.e. made into fibrils) such as beaten pulp (cellulosefibers mechanically squashed or cut in water or the like) can be alsoused. In this case, the outer diameter of the primary fiber can be 10 μmor more as long as that of each fibril is 1 μm (1000 nm) or less.

The amount of fibrous material added to a carbon raw material may bedetermined in consideration of intended physical properties of a finalproduct, production cost, the effect of suppressing expansion of amolten liquid depending on the shape of the fibrous carbon,dispersibility of the fibrous carbon, the relation between temperatureincrease rate and the degree of expansion of the molten liquid. In thecase that an organic fiber such as pulp and the like is used, decreasein the mass thereof may occur during the process due to carbonizationand accordingly, the amount of the fiber to be added is more than thatin case of fibrous carbon. Typically, it is sufficient that the amountof the fibrous material is not less than 0.05 mass % as a correspondingmass of the fibrous material heated at 800° C. in an inert atmospherefor 60 to 600 minutes. When the amount of the fibrous material is toolow, expansion of the molten liquid cannot be sufficiently suppressed.Addition of a fibrous material having an excellent conductivity is moreadvantageous since such a fiber lowers the contact resistance betweenthe resulted activated carbon particles.

As a conductive fiber, vapor grown carbon fiber is preferably used sincethe vapor grown carbon fiber contains carbon crystals grown along theaxis of each fiber filament of the fiber and contact resistance betweenactivated carbon particles can be effectively reduced. Vapor growncarbon fiber can be produced by feeding a gasified organic compound suchas benzene into a high-temperature atmosphere together with a transitionmetallic compound serving as a catalyst.

The vapor grown carbon fiber to be employed may be as-produced carbonfiber; carbon fiber which has undergone heat treatment at, for example,800 to 1,500° C.; or carbon fiber which has undergone graphitization at,for example, 2,000 to 3,000° C. However, as-produced carbon fiber orcarbon fiber which has undergone heat treatment at about 1,500° C. ismore preferred.

Preferably, the vapor grown carbon fiber employed in the presentinvention is branched carbon fiber. More preferably, each fiber filamentof the branched carbon fiber has a hollow structure in which a hollowspace extends throughout the filament, including a branched portionthereof, sheath-forming carbon layers of the filament assumeuninterrupted layers. As used herein, the term “hollow structure” refersto a structure in which a plurality of carbon layers form a sheath. Thehollow structure encompasses a structure in which sheath-forming carbonlayers form an incomplete sheath; a structure in which the carbon layersare partially broken; and a structure in which the laminated two carbonlayers are formed into a single carbon layer. The cross section of thesheath does not necessarily assume a round shape, and may assume anelliptical shape or a polygonal shape. No particular limitation isimposed on the interlayer distance (d002) of carbon crystal layers. Theinterlayer distance (d002) of the carbon layers as measured throughX-ray diffraction is preferably 0.339 nm or less, more preferably 0.338nm or less. The thickness (Lc) of the carbon crystal layer in the C axisdirection is 40 nm or less.

The outer diameter of each fiber filament of the vapor grown carbonfiber is 2 to 500 nm, and the aspect ratio of the filament is 10 to15,000. Preferably, the fiber filament has an outer diameter of 50 to500 nm and a length of 1 to 100 μm (i.e., an aspect ratio of 2 to2,000); or an outer diameter of 2 to 50 nm and a length of 0.5 to 50 μm(i.e., an aspect ratio of 10 to 25,000).

When the carbon material is activated with the alkali metal compound, aportion of low crystallinity contained in the surface of the carbonmaterial is corroded by the alkali metal compound. In the case where thecarbon fiber is mixed with the carbon material, while the surface of thecarbon material is corroded, the carbon fiber which is present in thevicinity of the carbon material is melt-bonded to the carbon material,and thus, the carbon fiber is melt-bonded onto the surfaces of theresultant activated carbon particles. Such melt-bonding of carbon fibersonto the surface of the activated carbon material reduces the contactresistance between the activated carbon particles and improves thecapacitance at a high current density.

The melt-bonded state include not only such a state where thecarbonaceous surface layer of a fibrous material, for example, carbonfiber and that of an activated carbon material are molten and bonded toeach other at points but also a state where those surface layers are notmolten but bonded to each other at their solid surfaces.

The amount of carbon fiber mixed with the carbon material is preferablynot less than 0.05 mass %, more preferably 0.1 to 50 mass %, mostpreferably 1 to 30 mass %. When the amount of the carbon fiber is lessthan 0.05 mass %, since the amount of the melt-bonded carbon fiber issmall, the effect of reducing the contact resistance between theactivated carbon particles is also small, the improvement incharge/discharge characteristics at high current density are notsufficiently obtained, and the effect of suppressing the expansion themolten liquid is small.

When the amount of carbon fiber to be mixed with is smaller than themixing ratio of the conductive material to the activated carbon forforming an electric double-layer capacitor, additional amount of carbonfiber may be added to the resulted activated carbon to form a capacitor.Alternatively, commonly used conductive material such as carbon blackmay be added.

In contrast, when the amount of carbon fiber to be mixed with is largerthan the normal mixing ratio of the conductive material to the activatedcarbon for forming an electric double-layer capacitor, suitable amountof carbon fiber may be added to a common activated carbon for enhancingelectric conductivity to form an electric double-layer capacitor.

For example, when the carbon fiber is added in an amount of 0.05 to 10mass % and the activation is effected, the product may be used for apolarizable electrode as it is, or alternatively, carbon fiber and/orcarbon black may be further added to the product to form a polarizableelectrode. When the amount of carbon fiber added exceeds 50 mass %, theactivated product may be added in an amount of 10 to 50 mass % of to 90to 50 mass % of the activated carbon to form a polarizable electrode.

No particular limitation is imposed on the alkali activation agent, solong as the agent is a compound containing an alkali metal. The presentinvention is effectively applied to substances which melt duringactivation. Preferred examples of the alkali activation agent includehydroxides, carbonates, sulfides, and sulfates of potassium, sodium, andcalcium. Specific examples of the alkali activation agent which may beemployed include potassium hydroxide, sodium hydroxide, ceciumhydroxide, potassium carbonate, sodium carbonate, potassium sulfide,sodium sulfide, potassium thiocyanate, potassium sulfate, and sodiumsulfate. Potassium hydroxide and sodium hydroxide are preferred, andpotassium hydroxide is more preferred. These compounds may be employedsingly or in combination of two or more species.

The amount of an alkali metal compound mixed with a carbonaceous rawmaterial may be determined in accordance with crystallinity of thecarbonaceous raw material, the amount of a surface functional group ofthe carbonaceous raw material, and the intended use of an activatedcarbon material to be produced. When crystallinity of the carbonaceousraw material is high and the amount of a surface functional group of theraw material is small, the amount of an alkali metal compound which isrequired tends to increase. For example, when potassium hydroxide isemployed, the ratio by mass of potassium hydroxide to the carbonaceousraw material is about 0.5 to about 7, preferably about 1 to about 5,more preferably about 2 to about 4. When the ratio by mass of potassiumhydroxide is less than 0.5, micropores are insufficiently developed,whereas when the ratio by mass is 7 or more, excessive activationproceeds and the walls of micropores are broken, so that the number ofmicropores decreases, and thus the specific surface area of theresultant activated carbon material tends to be reduced.

The activation temperature varies with the type and shape of a rawmaterial and the activation reaction rate. The activation temperature istypically 250 to 1,000° C., preferably 500° C. to 900° C., morepreferably 600° C. to 800° C. When the activation temperature is 400° C.or lower, activation proceeds insufficiently, the number of microporescontained in an activated carbon material becomes small, and capacitancewhen used as a polarizable electrode material in an electric doublelayer capacitor is lowered. When the activation temperature is 1,000° C.or higher, there arise problems, including shrinkage of microporescontained in an activated carbon material, considerable deterioration ofcharge characteristics at high current density, and corrosion of anactivation apparatus.

The temperature increase rate during activation may be determinedwhether fibrous carbon is added or not or in consideration of the amountof fibrous carbon to be added, and the degree of expansion of an alkalimolten liquid. When the temperature increase rate is high, the amount ofmoisture removed from an alkali metal compound per unit time increases,and the amount of gas (e.g., hydrogen gas) generated from the alkalimetal compound per unit time increases, and thus the alkali metalcompound tends to overflow a container. In contrast, when thetemperature increase rate is low, the yield of an activated carbonmaterial per container increases, but productivity is lowered. Thetemperature increase rate is typically 400 to 1° C./hour.

For example, when the temperature increase rate is 300° C./hour, thetotal volume of a carbonaceous raw material, fibrous carbon, and analkali metal compound to be placed in a container (crucible) is 15% thatof the entire volume of the container. When the temperature increaserate is lowered to 20° C./hour or less, the amount of gas generated perunit time can be reduced, and the yield of an activated carbon materialper container can be increased. However, from the viewpoint ofproductivity, preferably, the total amount of a carbonaceous rawmaterial, fibrous carbon, and an alkali metal compound to be placed in acontainer is increased to a possibly maximum level, and activation iscarried out at a high temperature increase rate.

As a result of activation, numerous basket-like micropores having a sizeof 2 to 5 nm (20 to 50 Å) are formed between several carbon layers, thenumber of micropores which has a radius of about 1 to about 2 nm (about10 to about 20 Å) and which is suitable for, for example, adsorption isincreased, and the adsorption volume of an activated carbon material isincreased.

The thus-produced activated carbon material exhibits high capacitance atthe first cycle of charge/discharge testing without application ofexcessive voltage, and exhibits high percent maintenance of thecapacitance.

When the activated carbon material was observed under a transmissionmicroscope, the material was found to contain no microcrystals similarto those of graphite, and to have merely a turbostratic structure asshown in FIG. 3. The ratio of the height of the D peak (1360 cm⁻¹) of aRaman spectrum of the material to the height of the G peak (1580 cm⁻¹)of the Raman spectrum (i.e., height from the base line to the peak pointas measured on the basis of the spectrum) in an observed curve was foundto be 0.8 to 1.2.

Here, the ratio of the intensity of the D peak to that of the G peak isused as an indicator to indicate the graphitization degree of carbonmaterial, and the intensity ratio expressed as the peak height ratiobecomes a smaller value as the graphitization degree is larger. Thevalue in an activated carbon containing microcrystals is generally onthe order of 0.6 and the present activated carbon containing nomicrocrystals exhibited values of 0.8 to 1.2.

Furthermore, an activated carbon black to be used in an electrode of acapacitor is required to have micropores of 20 to 50 Å, which isconsidered to attribute to the capacitance and the expansion of theelectrolyte, in a certain amount of more.

The BET specific surface area of the activated carbon material of thepresent invention determined by nitrogen adsorption method was found tobe 10 to 1,000 m²/g, which is smaller than that of the conventionallyobtained activated carbon materials (usually in the range of 2,000 to3,000 m²/g). The pore volume of pores having a size of 20 to 50 Å of theactivated carbon material of the present invention as measured by meansof a BJH (Barrett, Joyner and Halenda) method was found to be 0.02 ml/gor more.

Due to the crystalline structure and micropore structure, the activatedcarbon material exhibits a high capacitance from the firstcharge/discharge cycle without application of excessive voltage forintercalation of ions between graphite layers. In addition, conceivably,since the activated carbon material has undergone sufficientcarbonization, the amount of functional groups on the surface of thematerial is reduced, and lowering of capacitance can be prevented.

The tapping density of the thus produced activated carbon material wasmeasured by use of a tapping density meter (product of Kuramochi KagakuKikai Seisakusho), and found to be 0.35 to 0.70 g/ml (tapping: 50times). The powder resistance of the activated carbon material was foundto be 0.4 Ωcm or less at 1.0 MPa.

When vapor grown carbon fiber is added to the thus-produced activatedcarbon material, characteristics of the activated carbon material arefurther improved. Similar vapor grown carbon fiber as described abovecan be employed for that purpose. When the alkali activation has beeneffected after the addition of vapor grown carbon fiber, vapor growncarbon fiber can be added if required.

When the activated carbon material is mixed with vapor grown carbonfiber, contact resistance between activated carbon particles is reduced.Therefore, when a polarizable electrode is formed from the resultantmixture, the electrode exhibits enhanced strength and improveddurability.

Vapor grown carbon fiber to be employed may be as-produced carbon fiberwhich has undergone firing at 1,000 to 1,500° C., or carbon fiber whichhas undergone firing and then graphitization.

Vapor grown carbon fiber which has undergone gas activation or chemicalactivation may be employed. When such vapor grown carbon fiber isemployed, preferably, the surface of the carbon fiber may be controlledsuch that micropores (i.e., pores having a size of 20 Å or less)contained in the carbon fiber have a pore volume of 0.01 to 0.4 ml/g,and the carbon fiber has a BET specific surface area of 30 to 1,000m²/g. When carbon fiber containing a large number of micropores is mixedwith the activated carbon material, ion diffusion resistance in anelectrode formed from the resultant mixture increases.

The amount of vapor grown carbon fiber mixed with the activated carbonmaterial is preferably 0.02 mass % to 50 mass %, more preferably 0.05 to30 mass %. When the amount of the carbon fiber is 0.02 mass % or less,since contact points between the carbon fiber and activated carbonparticles increase insufficiently, satisfactory effects fail to beobtained. In contrast, when the amount of the carbon fiber is 50 mass %or more, the activated carbon content of a polarizable electrode islowered, resulting in lowering of capacitance.

A polarizable electrode and an electric double layer capacitor can beproduced from the activated carbon material of the present inventionthrough any known method. Specifically, a polarizable electrode isproduced through, for example, the following methods: a method in whichan electrically conductive agent and a binder are added to the activatedcarbon material, and the resultant mixture is subjected to kneading androlling; a method in which an electrically conductive agent, a binder,and if desired, a solvent are added to the activated carbon material tothereby prepare a slurry, and the resultant slurry is applied in apredetermined thickness to an electrically conductive material followedby removing the solvent by evaporation at room or elevated temperature;and a method in which a non-carbonized resin is mixed with the activatedcarbon material, and the resultant mixture is sintered. Examples of theelectrically conductive material usable for this purposes include a foilor a plate of aluminum, carbon-coated aluminum, stainless steel,titanium and the like having a thickness of about 10 μm to about 0.5 mm.

For example, if desired, an electrically conductive agent (e.g., carbonblack) is added to the powdery activated carbon material having anaverage particle size of about 5 to about 100 μm; a binder such aspolytetrafluoroethylene (PTFE) or polyvinylidene fluoride is added tothe resultant mixture; the resultant mixture is formed into a sheethaving a thickness of about 0.1 to about 0.5 mm; and the thus-formedsheet is dried under vacuum at a temperature of about 100 to about 200°C. Electrodes having a predetermined shape are formed from the sheetthrough punching. A metallic plate serving as a collector is laminatedon each of the electrodes, to thereby form a laminate. A separator issandwiched by the thus-formed two laminates such that the metallicplates are positioned outside, and the resultant laminate is immersed inan electrolytic solution, to thereby produce an electric double layercapacitor.

Any known electrolytic solution containing a non-aqueous solvent or anyknown water-soluble electrolytic solution may be employed as anelectrolytic solution for the electric double layer capacitor.

Examples of aqueous systems (aqueous electrolytic solutions) include asulfuric acid aqueous solution, a sodium sulfate aqueous solution, asodium hydroxide aqueous solution, a potassium hydroxide aqueoussolution, an ammonium hydroxide aqueous solution, a potassium chlorideaqueous solution, and a potassium carbonate aqueous solution.

A preferred non-aqueous system (non-aqueous electrolytic solution) isprepared from an organic solvent, and a quaternary ammonium salt or aquaternary phosphonium salt (i.e., electrolyte) containing a cationrepresented by R¹R²R³R⁴N⁺ or R¹R²R³R⁴P⁺ (wherein each of R¹, R², R³, andR⁴ represents a C1–C10 alkyl group or a C1–C10 allyl group) and an anionsuch as BF₄ ⁻, PF₆ ⁻, or ClO₄ ⁻. Examples of the organic solventsinclude ethers such as diethyl ether, dibutyl ether, ethylene glycolmonomethyl ether, ethylene glycol monoethyl ether, ethylene glycolmonobutyl ether, diethylene glycol monomethyl ether, diethylene glycolmonoethyl ether, diethylene glycol monobutyl ether, diethylene glycoldimethyl ether, and ethylene glycol phenyl ether; amides such asformamide, N-methylformamide, N,N-dimethyiformamide, N-ethylformamide,N, N-diethylformamide, N-methylacetamide, N,N-dimethylacetamide,N-ethylacetamide, N,N-diethylacetamide, N,N-dimethylpropionamide, andhexamethylphosphoryl amide; sulfur-containing compounds such as dimethylsulfoxide and sulfolan; dialkyl ketones such as methyl ethyl ketone andmethyl isobutyl ketone; cyclic ethers such as ethylene oxide, propyleneoxide, tetrahydrofuran, 2-methoxytetrahydrofuran, 1,2-dimethoxyethane,and 1,3-dioxolan; carbonates such as ethylene carbonate and propylenecarbonate; γ-butyrolactone; N-methylpyrrolidone; acetonitrile; andnitromethane. Preferred examples which may be employed includecarbonate-based non-aqueous solvents such as ethylene carbonate andpropylene carbonate. These electrolytes or solvents may be employed incombination of two or more species.

No particular limitation is imposed on the separator which, if desired,is provided between electrodes, so long as the separator is anion-permeable porous separator. Preferred examples of separators whichmay be employed include microporous polyethylene film, microporouspolypropylene film, polyethylene nonwoven fabric, polypropylene nonwovenfabric, glass-fiber-mixed nonwoven fabric, and glass mat filter.

MODE FOR CARRYING OUT THE INVENTION

The present invention will next be described in more detail by way ofillustrative examples, which should not be construed as limiting theinvention thereto. In the following examples, characteristics ofactivated carbon materials and electric double layer capacitors wereevaluated by means of the below-described methods.

(1) Measurement of BET Specific Surface Area and Pore Volume

BET specific surface area and pore volume were calculated by means of aBET method and a BJH method, on the basis of an adsorption isotherm ofnitrogen as measured at liquid nitrogen temperature by use of NOVA1200(product of Quantachrome Instruments). The amount of adsorbed nitrogenwas measured at a relative pressure (P/P0) of 0.01 to 1.0.

(2) Measurement of Raman Spectrum

The Raman spectrum of a carbon material serving as a raw material forproducing an activated carbon material was measured under the followingconditions: excitation light source: Ar laser (514.5 nm), detector:charge coupled device (CCD), slit width: 500 μm, exposure time: 60seconds.

(3) Capacitance

Polytetrafluoroethylene (PTFE) (10 parts by mass) and carbon black (10parts by mass) were added to an activated carbon material having anaverage particle size of 30 μm (80 parts by mass), the resultant mixturewas kneaded in an agate mortar, and the thus-kneaded product wassubjected to rolling by use of a roller, to thereby form a sheet havinga thickness of 0.5 mm. The thus-formed sheet was subjected to punchingto thereby form a disk having a diameter of 20 mm, and the disk wasdried at 200° C. under vacuum overnight. The resultant disk was employedas a polarizable electrode.

A cell for evaluation as shown in FIG. 1 was assembled from theaforementioned electrode in a glove box through which argon of highpurity was circulated. In FIG. 1, reference numeral 1 represents anupper lid formed of aluminum, 2 an O ring formed of fluorine rubber, 3collectors formed of aluminum, 4 an insulator formed of Teflon(registered trademark), 5 a container formed of aluminum, 6 a leafspring formed of aluminum, 7 polarizable electrodes, and 8 a separatorformed of glass fiber (thickness: 1 mm). LIPASTE-P/EAFIN (product ofTomiyama Pure Chemical Industries, Ltd.) (1 mol/liter), containingpropylene carbonate (PC) serving as a solvent and (C₂H₅)₄NBF₄ serving asan electrolyte, was employed as an electrolytic solution.

Charging and discharging were carried out at a current of 5 mA (1.6mA/cm²), 50 mA (16 mA/cm²) and 150 mA (48 mA/cm²) and 0 to 2.5 V or 0 to3.0 V by use of a charge/discharge test apparatus (HJ-101SM6, product ofHokuto Denko Co., Ltd.). A discharge curve obtained through the secondconstant-current discharging was used to calculate capacitance (F/g) ofan electric double layer capacitor per mass of activated carboncontained in the electrodes of the capacitor, along with capacitance(F/ml) of the capacitor per volume of the activated carbon.

Durability was evaluated on the basis of percent maintenance ofcapacitance (i.e., the ratio of capacitance after 20-cyclecharge/discharge testing to capacitance after the second-cyclecharging/discharging).

EXAMPLE 1

Coal pitch (product of Kawasaki Steel Corporation) having a softeningpoint of 86° C. was subjected to first-stage heat treatment at 500° C.and then to second-stage heat treatment at 700° C. The resultantcarbonaceous material was mixed with KOH such that the ratio by mass ofthe KOH to the coal pitch was 2.5, and the resultant mixture was placedin a crucible. The crucible was heated to 750° C. at 3° C./hour, and thetemperature of the crucible was maintained at 750° C. for 60 minutes, tothereby allow activation of the pitch to proceed. The thus-activatedcarbon material was washed with 1N hydrochloric acid, and then washedwith distilled water, to thereby remove residual KOH and metallicimpurities. The thus-washed carbon material was dried under vacuum at200° C., to thereby produce an activated carbon material.

The specific surface area of the activated carbon material was found tobe 930 m²/g. The pore volume of pores of the activated carbon materialhaving a size of 20 to 50 Å as measured by means of a BJH method wasfound to be 0.0416 ml/g. FIG. 2 shows a Raman spectrum of the activatedcarbon material. The ratio of the height of the D peak to that of the Gpeak was found to be 0.92.

When charging and discharging were performed at a current of 5 mA (1.6mA/cm²) and 2.5 V, the capacitance was found to be 36.5 F/g and 31.0F/ml, and the percent maintenance of capacitance after 20-cyclecharging/discharging was found to be 98.4%. When charging anddischarging were performed at a current of 5 mA (1.6 mA/cm²) and 3.0 V,the capacitance was found to be 37.7 F/g and 32.0 F/ml, and the percentmaintenance of capacitance after 20-cycle charging/discharging was foundto be 96.9%.

EXAMPLE 2

The activated carbon material produced through the method of Example 1was mixed with vapor grown carbon fiber (5 mass %), to thereby prepare apolarizable electrode material. When charging and discharging wereperformed at a current of 5 mA (1.6 mA/cm²) and 2.5 V, the capacitancewas found to be 36.4 F/g and 32.4 F/ml, and the percent maintenance ofcapacitance after 20-cycle charging/discharging was found to be 98.9%.When charging and discharging were performed at a current of 5 mA (1.6mA/cm²) and 3.0 V, the capacitance was found to be 39.5 F/g and 35.2F/ml, and the percent maintenance of capacitance after 20-cyclecharging/discharging was found to be 97.7%.

EXAMPLE 3

The procedure of Example 1 was repeated, except that coal pitch (productof Kawasaki Steel Corporation) having a softening point of 86° C. wassubjected to heat treatment at 500° C. and 800° C., to thereby producean activated carbon material.

The activated carbon material was employed as a polarizable electrodematerial. The specific surface area of the activated carbon material wasfound to be 173 m²/g. The pore volume of pores of the activated carbonmaterial having a size of 20 to 50 Å as measured by means of a BJHmethod was found to be 0.0271 ml/g. The ratio of the height of the Dpeak of a Raman spectrum of the activated carbon material to that of theG peak of the Raman spectrum was found to be 0.93.

The activated carbon material was observed under a transmission electronmicroscope (TEM) as shown in FIG. 3 and the activated carbon materialwas found to have no graphite structure and to have merely aturbostratic structure.

When charging and discharging were performed at a current of 5 mA (1.6mA/cm²) and 2.5 V, the capacitance was found to be 32.6 F/g and 31.9F/ml, and the percent maintenance of capacitance after 20-cyclecharging/discharging was found to be 98.7%. When charging anddischarging were performed at a current of 5 mA (1.6 mA/cm²) and 3.0 V,the capacitance was found to be 35.5 F/g and 34.8 F/ml, and the percentmaintenance of capacitance after 20-cycle charging/discharging was foundto be 97.2%.

EXAMPLE 4

The activated carbon material produced through the method of Example 3was mixed with vapor grown carbon fiber which had undergone alkaliactivation (pore volume of micropores: 0.3 ml, BET specific surfacearea: 530 m²/g) (5 mass %), to thereby prepare a polarizable electrodematerial. When charging and discharging were performed at a current of 5mA (1.6 mA/cm²) and 2.5 V, the capacitance was found to be 33.5 F/g and33.5 F/ml, and the percent maintenance of capacitance after 20-cyclecharging/discharging was found to be 99.0%. When charging anddischarging were performed at a current of 5 mA (1.6 mA/cm²) and 3.0 V,the capacitance was found to be 34.5 F/g and 34.5 F/ml, and the percentmaintenance of capacitance after 20-cycle charging/discharging was foundto be 98.0%.

COMPARATIVE EXAMPLE 1

Petroleum coke serving as a carbon material was mixed with KOH andplaced into a crucible such that the ratio by mass of the KOH to thecoke was 2.5 and the mixture was held at 750° C. for 60 minutes toeffect activation. The activated carbon material was washed with 1Nhydrochloric acid and then with distilled water to remove remaining KOHand metallic impurities. This was vacuum dried at 200° C. to obtainactivated carbon. The specific surface area of the activated carbonmaterial was found to be 1,905 m²/g. The ratio of the height of the Dpeak of a Raman spectrum of the activated carbon material to that of theG peak of the Raman spectrum was found to be 0.98.

When charging and discharging were performed at a current of 5 mA (1.6mA/cm²) and 2.5 V, the capacitance was found to be 44.5 F/g and 24.0F/ml, and the percent maintenance of capacitance after 20-cyclecharging/discharging was found to be 96.3%. When charging anddischarging were performed at a current of 5 mA (1.6 mA/cm²) and 3.0 V,the capacitance was found to be 45.0 F/g and 24.3 F/ml, and the percentmaintenance of capacitance after 20-cycle charging/discharging was foundto be 94.0%.

COMPARATIVE EXAMPLE 2

MCMB (mesocarbon microbeads, product of Osaka Gas Co., Ltd.) serving asa carbon material was mixed with KOH and placed into a crucible suchthat the ratio by mass of the KOH to the MCMB was 5 and the mixture washeld at 750° C. for 60 minutes to effect activation. The activatedcarbon material was washed with 1N hydrochloric acid and then withdistilled water to remove remaining KOH and metallic impurities. Thiswas vacuum dried at 200° C. to obtain activated carbon. The specificsurface area of the activated carbon material was found to be 127 m²/g.The pore volume of pores of the activated carbon material having a sizeof 20 to 50 Å was found to be 0.013 ml/g. The ratio of the height of theD peak of a Raman spectrum of the activated carbon material to that ofthe G peak of the Raman spectrum was found to be 0.92.

When charging and discharging were performed at a current of 5 mA (1.6mA/cm²) and 2.5 V, the capacitance was found to be 10.2 F/g and 9.4F/ml, and the percent maintenance of capacitance after 20-cyclecharging/discharging was found to be 99.1%. When charging anddischarging were performed at a current of 5 mA (1.6 mA/cm²) and 3.0 V,the capacitance was found to be 11.5 F/g and 10.6 F/ml, and the percentmaintenance of capacitance after 20-cycle charging/discharging was foundto be 98.5%.

EXAMPLE 5

Coal pitch (product of Kawasaki Steel Corporation) having a softeningpoint of 86° C. was subjected to first-stage heat treatment at 500° C.and then to second-stage heat treatment at 700° C. The resultantcarbonaceous material was mixed with KOH such that the ratio by mass ofthe KOH to the coal pitch was 2.5 and with vapor grown carbon fiber(fiber diameter: 50–500 nm, fiber length: about 20 μm) such that theratio by mass of the carbon fiber to the coal pitch was 0.05, and theresultant mixture was placed in a crucible up to a height of 80 mm fromthe bottom. The crucible was heated to 750° C. at a temperature increaserate of 350° C./hour, and the temperature of the crucible was maintainedat 750° C. for 60 minutes, to thereby allow activation of the pitch toproceed. The thus-activated carbon material was washed with 1Nhydrochloric acid, and then washed with distilled water, to therebyremove residual KOH and metallic impurities. The thus-washed carbonmaterial was dried under vacuum at 200° C., to thereby produce anactivated carbon material.

In a case where vapor grown carbon fiber was not used, an alkali moltenliquid rose up to a height of 560 mm from the bottom of the crucible. Inthis Example, by using vapor grown carbon fiber, an alkali molten liquidrose up no higher than a height of 120 mm (1.5 times as high as thefilling height of the mixture) from the bottom of the crucible, andproductivity was drastically improved.

The specific surface area of the activated carbon material was found tobe 930 m²/g. The pore volume of pores of the activated carbon materialhaving a size of 20 to 50 Å as measured by means of a BJH method wasfound to be 0.0450 ml/g. The ratio of the height of the D peak to thatof the G peak was found to be 0.90.

When charging and discharging were performed at a current of 5 mA (1.6mA/cm²) and 2.5 V, the capacitance was found to be 37.0 F/g and 31.5F/ml, and the percent maintenance of capacitance after 20-cyclecharging/discharging was found to be 99.5%. When charging anddischarging were performed at a current of 5 mA (1.6 mA/cm²) and 3.0 V,the capacitance was found to be 38.0 F/g and 32.3 F/ml, and the percentmaintenance of capacitance after 20-cycle charging/discharging was foundto be 98.2%. As compared to Example 1 where vapor grown carbon fiber wasnot used, the percent maintenance of capacitance was improved.

EXAMPLE 6

A phenol resin (trade name: Bellpearl R800, product of Kanebo, Ltd.) wascarbonized in a nitrogen atmosphere at 700° C. for four hours. Thethus-carbonized product (150 g), vapor grown carbon fiber (average fiberdiameter: about 500 nm, fiber length: about 20 μm) (7.5 g), andpotassium hydroxide pellets (473 g) were placed in a metallic crucible(100 mmφ×530 mm). The thickness of a layer of the mixture of thecarbonized product, vapor grown carbon fiber, and potassium hydroxidewas found to be 80 mm. The crucible was placed in an electric furnaceand heated to 750° C. at a temperature increase rate of 350° C./hr undera nitrogen stream, and the temperature of the crucible was maintained at750° C. for 30 minutes. After completion of activation, the crucible wasremoved from the furnace, and then visually observed. The resultsrevealed that an alkali molten liquid rose up to a height of 130 mm fromthe bottom of the crucible. The thus-activated carbon material waswashed with water and 1N hydrochloric acid, and then washed withdistilled water, to thereby remove residual alkali and metallicimpurities. After the thus-washed activated carbon material was dried,BET specific surface area and capacitance were measured. The BETspecific surface area was found to be 2,335 m²/g; and the capacitancewas found to be 42.9 F/g (at 1.6 mA/cm²), 36.7 F/g (at 16 mA/cm²), 24.7F/g (at 48 mA/cm²), and 26.7 F/ml (at 1.6 mA/cm²).

The obtained activated carbon was observed under a transmission electronmicroscope (TEM) as shown in FIG. 4 and the activated carbon materialwas found to have a spherical shape on the surface of which carbonfibers are melt-bonded.

EXAMPLE 7

A phenol resin (trade name: Bellpearl R800, product of Kanebo, Ltd.) wascarbonized in a nitrogen atmosphere at 700° C. for four hours. Thethus-carbonized product (150 g), vapor grown carbon fiber (average fiberdiameter: about 500 nm, fiber length: about 20 μm) (15 g), and potassiumhydroxide pellets (495 g) were placed in a metallic crucible (100mmφ×530 mm). The thickness of a layer of the mixture of the carbonizedproduct, vapor grown carbon fiber, and potassium hydroxide was found tobe 85 mm. The crucible was placed in an electric furnace and heated to750° C. at a temperature increase rate of 350° C./hr under a nitrogenstream, and the temperature of the crucible was maintained at 750° C.for 30 minutes. After completion of activation, the crucible was removedfrom the furnace, and then visually observed. No rise of an alkalimolten liquid was observed. The thus-activated carbon material waswashed with water and 1N hydrochloric acid. After the thus-washedactivated carbon material was dried, BET specific surface area andcapacitance were measured. The BET specific surface area was found to be2,400 m²/g; and the capacitance was found to be 40.6 F/g (at 1.6mA/cm²), 34.5 F/g (at 16 mA/cm²), 23.3 F/g (at 48 mA/cm²), and 25.9 F/ml(at 1.6 mA/cm²).

EXAMPLE 8

A phenol resin (trade name: Bellpearl R800, product of Kanebo, Ltd.) wascarbonized in a nitrogen atmosphere at 700° C. for four hours. Thethus-carbonized product (150 g), wood beaten pulp (48 g), and potassiumhydroxide pellets (472 g) were placed in a metallic crucible (100mmφ×530 mm). The thickness of a layer of the mixture of the carbonizedproduct, pulp, and potassium hydroxide was found to be 170 mm. Thecrucible was placed in an electric furnace and heated to 750° C. at atemperature increase rate of 350° C./hr under a nitrogen stream, and thetemperature of the crucible was maintained at 750° C. for 30 minutes.After completion of activation, the crucible was removed from thefurnace, and then visually observed. The results revealed that an alkalimolten liquid rose up to a height of 190 mm from the bottom of thecrucible. The thus-activated carbon material was washed with water, 1Nhydrochloric acid and then with distilled water to thereby removeresidual alkali and metallic impurities. After the thus-washed activatedcarbon material was dried, BET specific surface area and capacitancewere measured. The BET specific surface area was found to be 1,836 m²/g;and the capacitance was found to be 32.8 F/g (at 1.6 mA/cm²) and 24.6F/ml (at 1.6 mA/cm²).

EXAMPLE 9

A phenol resin (trade name: Bellpearl R800, product of Kanebo, Ltd.) wascarbonized in a nitrogen atmosphere at 700° C. for four hours. Thethus-carbonized product (150 g), wood beaten pulp carbonized at 700° C.(7.5 g), and potassium hydroxide pellets (472 g) were placed in ametallic crucible (100 mmφ×530 mm). The thickness of a layer of themixture of the carbonized product, pulp, and potassium hydroxide wasfound to be 75 mm. The crucible was placed in an electric furnace andheated to 750° C. at a temperature increase rate of 350° C./hr under anitrogen stream, and the temperature of the crucible was maintained at750° C. for 30 minutes. After completion of activation, the crucible wasremoved from the furnace, and then visually observed. The resultsrevealed that an alkali molten liquid rose up to a height of 170 mm fromthe bottom of the crucible. The thus-activated carbon material waswashed with, 1N hydrochloric acid and then with distilled water tothereby remove residual alkali and metallic impurities. After thethus-washed activated carbon material was dried, BET specific surfacearea and capacitance were measured. The BET specific surface area wasfound to be 2,151 m²/g; and the capacitance was found to be 39.8 F/g (at1.6 mA/cm²) and 25.6 F/ml (at 1.6 mA/cm²).

COMPARATIVE EXAMPLE 3

A phenol resin (trade name: R800, product of Kanebo, Ltd.) wascarbonized in a nitrogen atmosphere at 700° C. for four hours. Thethus-carbonized product (150 g) and potassium hydroxide pellets (450 g)were placed in a metallic crucible (100 mmφ×530 mm). The thickness of alayer of the mixture of the carbonized product and potassium hydroxidewas found to be 70 mm. The crucible was placed in an electric furnaceand activation was effected as in Example 6. After completion ofactivation, the crucible was removed from the furnace, and then visuallyobserved. The results revealed that an alkali molten liquid rose up to aheight of 490 mm from the bottom of the crucible.

The thus-activated carbon material was washed with 1N hydrochloric acidand then with distilled water to thereby remove residual alkali andmetallic impurities.

The product was evaluated as in Example 6. The capacitance was found tobe 39.4 F/g (at 1.6 mA/cm²), 31.7 F/g (at 16 mA/cm²) and 17.9 F/g (at 48mA/cm²).

COMPARATIVE EXAMPLE 4

A phenol resin (trade name: R800, product of Kanebo, Ltd.) wascarbonized in a nitrogen atmosphere at 700° C. for four hours. To thethus-carbonized product (150 g) carbon fiber chop (KrecaChop M-101S;fiber diameter: 14.5 μm) (7.5 g) was added and after potassium hydroxidepellets (473 g) were added thereto the mixture was mixed well and placedin a inconel crucible (100 mmφ×530 mm). The thickness of a layer of themixture of the carbonized product and potassium hydroxide was found tobe 70 mm. The crucible was placed in an electric furnace and activationwas effected as in Example 6. After completion of activation, thecrucible was removed from the furnace, and then visually observed. Theresults revealed that an alkali molten liquid rose up to a height of 530mm from the bottom of the crucible, and thus no effect of suppressingthe expansion of the molten liquid was observed.

INDUSTRIAL APPLICABILITY

The method of the present invention comprising two-stage heat treatmentof a coal-based pitch at different temperature ranges and activatingwith an alkali enables production of an activated carbon materialexhibiting excellent durability and high capacitance (F/ml) withoutapplication of excessive voltage.

When the activated carbon material is mixed with vapor grown carbonfiber, a polarizable electrode and an electric double layer capacitorexhibiting more excellent characteristics can be produced.

By adding fibrous carbon to a reactant (i.e., a composition containingthe carbonaceous raw material and the alkali metal compound), expansionof an alkali molten liquid can be suppressed during activation, andproductivity can be improved.

Furthermore, employment of an fibrous carbon material which is excellentin conductivity as a fibrous material, activated carbon material onwhich carbon fiber is melt-bonded can be produced, which enablesproduction of an electric double layer capacitor and a polarizableelectrode exhibiting excellent charge/discharge characteristics at highcurrent density.

1. A method for producing an activated carbon material, wherein themethod comprises a step of thermally treating coal-based pitch with atwo-stage heat treatment at two temperature ranges of 400° C. to 600° C.and 600° C. to 900° C.; and a step of mixing and heating thethus-treated coal-based pitch with an alkali metal compound for theactivation thereof, and wherein the step of thermally treatingcoal-based pitch at two temperature ranges is carried out in a vapor ofan alkali metal.
 2. The method for producing an activated carbonmaterial as claimed in claim 1, wherein the alkali metal compound is atleast one alkali hydroxide selected from the group consisting of sodiumhydroxide, potassium hydroxide, and cesium hydroxide.
 3. The method forproducing an activated carbon material as claimed in claim 1, whereinthe two temperature ranges are 450° C. to 600° C. and 600° C. to 900° C.4. The method for producing an activated carbon material as claimed inclaim 1, wherein the alkali metal compound is at least one speciesselected from the group consisting of potassium, sodium, and cesiumcompounds.
 5. The method for producing an activated carbon material asclaimed in claim 1, wherein the step for the activation comprises addinga fibrous material to the coal-based pitch.
 6. The method for producingan activated carbon material as claimed in claim 5, wherein the amountof the fibrous material is not less than 0.05 mass % as a correspondingmass of the fibrous material heated at 800° C. in an inert atmosphere onthe basis of the coal-based pitch.
 7. The method for producing anactivated carbon material as claimed in claim 5, wherein the outerdiameter of each fiber filament of the fibrous material is 1000 nm orless.
 8. The method for producing an activated carbon material asclaimed in claim 5, wherein the fibrous material is a material capableof maintaining its shape up to at least 300° C.
 9. The method forproducing an activated carbon material as claimed in claim 5, whereinthe fibrous material is at least one species selected from the groupconsisting of a fibrous carbon, carbonized material of organic fiber,unmeltable fiber, beaten pulp and cellulose fiber.
 10. The method forproducing an activated carbon material as claimed in claim 9, whereinthe fibrous carbon is at least one species selected from the groupconsisting of a carbon nano tube, whiskers, vapor grown carbon fiber,carbon fiber ribbon and coiled carbon fiber.
 11. The method forproducing an activated carbon material as claimed in claim 10, whereineach fiber filament of the vapor grown carbon fiber contains a hollowspace extending along its center axis, and has an outer diameter of 2 to500 nm and an aspect ratio of 10 to 15,000.
 12. The method for producingan activated carbon material as claimed in claim 11, wherein the vaporgrown carbon fiber is branched carbon fiber.
 13. A method for producingan activated carbon material, wherein the method comprises adding analkali metal compound as an activating agent and a fibrous material to acarbonaceous raw material to form a mixture and heating the mixture toactivate the carbonaceous raw material, wherein the fibrous materialcomprises filaments having an outer diameter of 1000 nm or less orfibrils having an outer diameter of 1000 nm or less.
 14. The method forproducing an activated carbon material as claimed in claim 13, whereinthe amount of the fibrous material is not less than 0.05 mass % as acorresponding mass of the fibrous material heated at 800° C. in an inertatmosphere on the basis of the carbonaceous raw material.
 15. The methodfor producing an activated carbon material as claimed in claim 13,wherein the outer diameter of each fiber filament of the fibrousmaterial is 1000 nm or less.
 16. The method for producing an activatedcarbon material as claimed in claim 13, wherein the fibrous material isa material capable of maintaining its shape up to at least 300° C. 17.The method for producing an activated carbon material as claimed inclaim 13, wherein the fibrous material is at least one species selectedfrom the group consisting of a fibrous carbon, carbonized material oforganic fiber, unmeltable fiber, beaten pulp and cellulose fiber. 18.The method for producing an activated carbon material as claimed inclaim 17, wherein the fibrous carbon is at least one species selectedfrom the group consisting of a carbon nano tube, whiskers, vapor growncarbon fiber, carbon fiber ribbon and coiled carbon fiber.
 19. Themethod for producing an activated carbon material as claimed in claim18, wherein each fiber filament of the vapor grown carbon fiber containsa hollow space extending along its center axis, and has an outerdiameter of 2 to 500 nm and an aspect ratio of 10 to 15,000.
 20. Themethod for producing an activated carbon material as claimed in claim19, wherein the vapor grown carbon fiber is branched carbon fiber, andeach fiber filament of the branched carbon fiber contains a hollow spaceextending throughout the filament, including a branched portion thereof.21. A method for producing an activated carbon material, wherein themethod comprises a step of thermally treating coal-based pitch with atwo-stage heat treatment at two temperature ranges of 400° C. to 600° C.and 600° C. to 900° C.; and a step of mixing and heating thethus-treated coal-based pitch with an alkali metal compound for theactivation thereof, wherein a softening point of the coal-based pitch is100° C. or lower.
 22. The method for producing an activated carbonmaterial as claimed in claim 21, wherein the temperature increase rateis 3 to 10° C./hour in the thermally treating step in the range of 400°C. to 600° C.
 23. The method for producing an activated carbon materialas claimed in claim 21, wherein the step for the activation comprisesadding a fibrous material to the coal-based pitch.
 24. The method forproducing an activated carbon material as claimed in claim 23, whereinthe amount of the fibrous material is not less than 0.05 mass % as acorresponding mass of the fibrous material heated at 800° C. in an inertatmosphere on the basis of the coal-based pitch.
 25. The method forproducing an activated carbon material as claimed in claim 23, whereinthe outer diameter of each fiber filament of the fibrous material is1000 nm or less.
 26. The method for producing an activated carbonmaterial as claimed in claim 23, wherein the fibrous material is amaterial capable of maintaining its shape up to at least 300° C.
 27. Themethod for producing an activated carbon material as claimed in claim23, wherein the fibrous material is at least one species selected fromthe group consisting of a fibrous carbon, carbonized material of organicfiber, unmeltable fiber, beaten pulp and cellulose fiber.
 28. The methodfor producing an activated carbon material as claimed in claim 27,wherein the fibrous carbon is at least one species selected from thegroup consisting of a carbon nano tube, whiskers, vapor grown carbonfiber, carbon fiber ribbon and coiled carbon fiber.
 29. The method forproducing an activated carbon material as claimed in claim 28, whereineach fiber filament of the vapor grown carbon fiber contains a hollowspace extending along its center axis, and has an outer diameter of 2 to500 nm and an aspect ratio of 10 to 15,000.
 30. The method for producingan activated carbon material as claimed in claim 29, wherein the vaporgrown carbon fiber is branched carbon fiber.